算法模板目录 ：个人算法模板

using ULL = unsigned long long;
const ULL M = (1ull << 61) - 1;
ULL add(ULL a, ULL b) {
    a += b;
    if (a >= M) {
        a -= M;
    }
    return a;
}
ULL mul(ULL a, ULL b) {
    __uint128_t c = __uint128_t(a) * b;
    return add(c >> 61, c & M);
}